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Source : https://support.apple.com/kb/ht1782
Learn about using Disk Utility to verify or repair disks.
Disk Utility can verify your computer’s startup disk (volume) without starting up from another volume. This feature is called “Live Verification.” If Disk Utility discovers any issues that require a repair, you will need to start up from your Mac OS X Install DVD and use Disk Utility on that disc to make repairs (You can’t repair your startup volume while your computer is started from it.).
Important things to remember
- Live Verification only works on Mac OS X Extended (Journaled)-formatted (HFS+J) volumes. If you try to verify a non-journaled disk, Disk Utility will display this message: “ERROR: could not freeze volume (Operation not supported).”
- During a Live Verification, the Disk Utility progress indicator may stop advancing, open applications may act slow or become unresponsive, the progress pointer (it looks like a spinning pinwheel) may appear, or your computer could appear to stop responding (“freeze”).
- Once you start a Live Verification, you may not be able to cancel it, depending on how far along it has progressed.
- You may experience some issues if you try to verify or repair any unmountable, non-startup disk.
- If you see any “Incorrect size for file temp” alerts, you can safely ignore them.
Issues may occur if you try to verify or repair unmountable disks
Disk Utility may stop responding without displaying an error message, or stall your computer for several seconds, when trying to verify or repair some non-startup volumes that can’t be unmounted.
If you try to verify a volume that is not your startup disk but for some reason Disk Utility can’t unmount the volume (for example, the disk may have open files), the verification will appear to start but then stop without displaying any alert message. If you look in the Console (/Applications/Utilities/), you will see an entry like this:
Verifying volume “Storage”
The disk “Storage” could not be unmounted
Could not unmount disk for verification, attempting live verify
If you try to repair a disk that cannot be unmounted, the repair will appear to start, but then stop as Disk Utility displays this message: “Repairing disk failed with error. Could not unmount disk.”
If you look in the Console (/Applications/Utilities/), you will see an entry like this:
Verify and Repair disk “Storage”
The disk “Storage” could not be unmounted
Could not unmount disk for verification
Repairing disk failed with error. Could not unmount disk
In some rare situations, your computer might not respond for several seconds.
If you cannot determine which files are open on the disk that you want to verify or repair, restart your computer and then mount the disk again, or start up from your Mac OS X Install DVD or CD to repair.
“Incorrect size for file temp” alerts can be safely ignored
You may notice some “Incorrect size for file tempnumber” alerts when you attempt to verify or repair a volume using Disk Utility or fsck_hfs with the “-l” option. You can safely ignore these alerts for any “tempnumber” files.
For example, you might see something like this:
Verifying volume “Macintosh HD”
Checking HFS Plus volume.
Checking Extents Overflow file.
Checking Catalog file.
Incorrect size for file temp420595
(It should be 0 instead of 84538)
Incorrect size for file temp468627
(It should be 0 instead of 16464)
Checking multi-linked files.
Checking Catalog hierarchy.
Checking volume bitmap.
Checking volume information.
The volume Macintosh HD needs to be repaired.
Error: The underlying task reported failure on exit
1 HFS volume checked
Volume needs repair
If this happens, use fsck in single user mode, or start up your computer from a different volume before verifying or repairing.
Advanced: This issue can happen because the on-disk size for truncated open-unlinked files doesn’t get updated before you start a live verification. The presence of these files doesn’t cause an issue because their in-memory size is correct. These files are deleted as soon as they are closed. If your computer does not shut down normally, they will be deleted during the next startup.